1. Field of the Invention
This invention pertains to high speed modulation of light. More particularly, this invention concerns apparatuses that modulate light through surface plasmon wave interaction.
2. Description of the Prior Art
A surface plasmon wave (SPW) is an electromagnetic wave supported by the interface between a metal and a dielectric material. Metal and all conductors can be defined as a gas of electrons in statistical equilibrium inside a continuum of positive fixed charges. It is a condensed electronic plasma with electronic charge density approximately equal to 10.sup.23 electrons per cm.sup.3. Plasmons are natural energy quanta associated with the collective electronic charge oscillations in the metal. Because of high electron density, quantum effects dominate. Such waves are generally discussed in A. D. Boardman, ed., "Electromagnetic Surface Modes", John Wiley Pub. (1982), incorporated by reference herein.
Surface plasmon waves can be optically excited by resonant coupling. The condition for resonance is strongly dependent on the refraction indices and thicknesses of the media near the metal-dielectric interface. The intensity of the light wave can be modulated by coupling the light wave with the surface plasmon wave. Typically, if coupling between the surface plasmon wave and the light wave is strong, the light wave attenuation is strong, and if coupling is weak the light wave propagates with little or no attenuation. Attenuated total reflection (ATR) effect has been utilized to optically excite surface plasmon waves through a high-index prism. Light, traveling in free-space, is sent toward the metal-dielectric interface through the prism with an angle larger than the critical angle, producing an evanescent field which may overlap the surface plasmon wave field. Two configurations have been used for optically exciting surface plasmon waves in a bulk-optic arrangement. The first is Otto's ATR configuration. In Otto's configuration there is a small air gap between a high-index prism and a thin dielectric layer deposited on a thick metal substrate. Otto's configuration can be seen in FIG. 1 and is discussed further in A. Otto, "Excitation of Nonradiative Surface Plasma Waves in Silver by the Method of Frustrated Total Reflection", 216 Z. Phys. 398-410 (1968) incorporated by reference herein. Another configuration used to optically excite surface plasmon waves is Kretschmann's modified ATR configuration as shown in FIG. 2. In this configuration, a thin metallic foil is sandwiched between a prism and a dielectric substrate. This configuration is more practical since there is no air gap. This type of configuration is disclosed in U.S. Pat. No. 4,451,123. Schildkraut, "Long-Range Surface Plasmon Electro-optic Modulator", 27 Appl. Opt. 4587 (1988) and E. Kretschmann, "Die Bestimmung Optischer Konstanten von Metallen durch Anreguns von Oberflachenplasma-schwingungen", 241 Z phys. 313-24 (1971), incorporated by reference herein, are also of interest for their discussion of such prism configurations.
Surface plasmon wave effects can be discussed from two viewpoints. The first is the quantum mechanical viewpoint which deals with photons, electrons, plasmons, and other particles and the interaction between those particles. The energy of such particles can be described by the equation EQU E=hv. (Eq. 1)
This equation describes the energy and the momentum of particles. The interaction between light and metallic mediums can be understood by considering the gas-like nature of electrons in metals (fermi gases) and by considering the electrons as plasmas, i.e., media containing charged particles (ions).
Plasmons have resonant frequencies. If a plasmon is stimulated by radiation at its resonant frequency, resonance is observed and some of the energy of the stimulating radiation is absorbed by the plasmon. So from the quantum particle point of view, surface plasmon wave systems are discussed in the context of the interaction of particles in a plasma having certain resonances.
The other view point from which to discuss surface plasmon waves involves Maxwell's equations, materials constants, and boundary conditions. In a simple model, metal can be characterized by a complex dielectric constant according to the following equation EQU .epsilon..sub.m =.epsilon..sub.real +i.epsilon..sub.imag (Eq. 2), ##EQU1## where m is the electron mass and n=10.sup.23. Thus the following obtains: EQU f.sub.p bulk=3.times.10.sup.15 Hz or .lambda..sub.p bulk=0.1 um(Eq. 5)
In surface plasmon wave technology, it is not the bulk plasmon that is important but rather the surface plasmon, the latter describing electron behavior at the metal boundary. The natural oscillation frequencies of surface electrons are a strong function of both the surface geometry and the interfaced dielectric medium. For planar boundaries and simple metals, the surface plasmon frequency is related to the bulk frequency by ##EQU2## In general the resonant optical wavelengths (frequencies) of the surface plasmon can be adjusted within a broad spectrum including UV, VIS, and nIR. This is in contradistinction to the bulk wavelength which is located in the UV area only, at about 0.1 um. In the state of the art, surface plasmon effects are realized by using the Kretschmann configuration wherein the attenuated total reflection effect has been utilized to optically excite SPWs through a prism. Note that in general the surface plasmon wave cannot be excited by direct coupling because momentum and energy conservation laws cannot be satisfied simultaneously. In the Kretschmann configuration, however, both conservation laws are satisfied.
A numerical example of the SPW mode index is shown in FIG. 3 which is a table showing two metal types, aluminum and gold, the former having a higher index of refraction, and two dielectric types, air and glass, the latter having a higher dielectric constant. From FIG. 3 it can be seen that the real part of the SPW mode index is larger than the dielectric index, the imaginary part of the SPW mode index is smaller with a metal having a lower index of refraction such as gold, and for a given metal the imaginary part of the SPW mode index is larger with the dielectric of the higher index. FIG. 4 shows the metal-dielectric interface along the Z-axis having a surface plasmon wave whose wavelength in vacuum, .lambda..sub.o, equals 0.83 um.
With respect to surface plasmon wave phase velocity and propagation loss, the surface plasmon wave normalized propagation constant (mode index) can be represented by a complex quantity. EQU n.sub.SPW =n.sup.R.sub.SPW +in.sup.I.sub.SPW (Eq. 7)
where n.sup.R.sub.SPW and n.sup.I.sub.SPW are, respectively, the real and imaginary parts. The surface plasmon wave phase velocity is given by ##EQU3## where c is the speed of light in vacuum.
SPW propagation loss per unit length L is determined by the imaginary part of the propagation constant. In decibel units, we have loss EQU (db)=10 log.sub.e -2k n.sup.I SPW.times.L (Eq. 9) EQU or loss ##EQU4## where .lambda..sub.0 is the light wavelength in vacuum.
As mentioned above, surface plasmon waves in a smooth plain boundary cannot be excited optically by direct coupling. This is because the momentum and energy conservation laws cannot be satisfied simultaneously. This can be seen in the w-k diagram for the dielectric medium and SPW, where there is no intersection between the two curves as shown in FIG. 5. For a given frequency, the SPW momentum exceeds the light momentum in the dielectric. Direct coupling using rough surfaces is possible but difficult to analyze. Evanescent wave coupling, as discussed above in the context of Otto's and Kretschmann's couplers, is necessary.
State of the art high speed modulators include the multiplex quantum well (MQW). MQW modulators offer extremely high frequency (greater than 5 GHz) with very short interaction lengths (microns), fairly low drive voltages (less than 10 volts), and moderate insertion loss (less than 3 dB) in the visible through near IR (nIR) spectrum. MQW modulators have been applied to a broad variety of optical signal processing, computing, storage, and communication schemes which require fast modulation or switching. The MQW modulator is comprised of numerous thin semiconductor layers which, when a voltage applied thereto is varied, varies the intensity of light passing through the modulator. MQW modulators use semi-transparent semiconductor material. Unfortunately, MQW techniques are difficult to implement, very expensive, and may even be inapplicable in extreme environmental conditions.
Recent progress in long distance fiber optic communication has stimulated the development of a variety of switching techniques for communication networks, local area networks (LAN), high power laser beam repetition, spatial light modulation (SLM), and external LD modulation. These techniques, the applications of which are more universal than those of MQWs, have been applied not only to fiber optic communication but also to optical computer, sensing, high power laser printing, laser surgery, etc. These other types of technologies include magneto-optic SLMs which are relatively slow (kHz), exhibit high electric power consumption, and high optical material loss. Examples of magneto-optic SLMs include those produced by Semetex. Another is liquid crystal (LC) SLMs which are relatively fast, and are based on ferroelectric LCs with a 20 ns (or longer) maximum response time. Yet another is a photo refractive switch which is based on LiNbO.sub.3 such as those produced by Crystal Technology, Inc. or based on PLZT, the latter having approximately 70 ns or more response times. MQW modulators which are very fast (greater than one picosecond) are discussed above. Finally there is the class of plasmon modulators which are super fast (greater than 5 picoseconds).
The latter two types of modulation technology--MQW and plasmon modulators--have a fundamental advantage over the first four switching systems, namely at least three orders of magnitude higher speed. In addition, the latter two technologies involve lower power consumption and drive voltage. The surface plasmon wave technology, however, has certain important advantages over the MQW technology. First, the wavelength range of the SPW technology extends from near IR to visible. MQW technology is near IR. Both technologies have about the same modulation frequency, potentially greater than 100 GHz. Likewise, the interaction lengths of MQW and SPW technology are both rather short, .about.100 um.
With regard to interaction length discussed above, interaction length is the distance over which the material of the modulator interacts with the light beam. In an ideal case the on-state of a modulator would fully absorb the light beam, and the off-state would fully transmit the light beam. How close a modulator system comes to the ideal case can be expressed in the form of the extension ratio (ER), where in the ideal case ER equals infinity. EQU ER=off-state intensity/on-state intensity. (Eq. 11)
In the alternative ER can be expressed as EQU ER=W=T.sub.1 /T.sub.2 (Eq. 12)
T.sub.1 is the fully off-state (value 1), and T.sub.2 is the fully onstate (value 0) as shown in FIG. 6. Also in the ideal case the speed, or modulation frequency, will be as great as possible. Typically, however, real systems have limited W and speed. If the interaction length, L, is short, T.sub.1 is low in the usual case. If the interaction length, L, is very long, it is usually the case that T.sub.1 is high (which makes the modulator system bulky and inefficient). A moderate interaction length can be measured in millimeters, while a short interaction length is less than a millimeter.
The drive voltage of MQW technology is higher than SPW technology. MQW voltage levels are -10 volts and SPW voltage levels are less than one volt. Lower drive voltages mean less power consumption. Insertion loss for the SPW system is only -3 db per meter, whereas insertion losses for MQW are -3 db per centimeter. Insertion loss is a measurement of off-state absorption. Note that the insertion loss of SPW modulators is measured on the meter scale.
The cost of making MQW modulators is very high, while the cost of the SPW technology is low. The cost of MQW technology is high in part because it requires sophisticated GaAs VLSI processing. Furthermore, the laser damage threshold of SPW modulators is extremely high, while that of MQW technology is low. All semiconductor devices, including MQW devices, have poor laser damage thresholds and are highly temperature sensitive. Similarly, ruggedization and environmental stability is low for MQW modulators and high for SPW modulators.
Lastly, the basic material used in MQW systems is GaAs, while the basic material for SPW technology is polymer-glass. Thus, from a materials point of view, MQW technology is closely related to laser diode (LD) technology which is likewise semiconductor based, while the SPW technology is dielectric based and thus closely related to low insertion loss fiber-optic media technology. Therefore, in contrast to MQWs, SPWs can also be used in high power laser switching and modulation with very high laser damage thresholds, greater than 1 GW per cm.sup.2.
Generally, in optoelectronics two basic trends have emerged. The first follows printed circuit GaAs VLSI electronics technology which is heavily semiconductor based. Refractive indices of these materials are in the range of about n=2-4. The second, and more recent, trend follows glass-polymer technology. The general advantage of this technology is that glass and polymers are compatible with most optical materials, have very low absorption, and can be used with many adhesives and recording materials having a low index of refraction. One problem that low index of refraction (n=1.5-1.6) presents is high Fresnel reflection losses if two substantially different materials (say, polymer and GaAs) are used. Antireflection coatings, however, lessen this effect. The basic advantage of the first trend, which follows printed circuit VLSI technology, is that this technology is compatible with most light sources such as GaAs sources and LDs. Thus, it can be said that MQW technology is light source compatible and that SPW technology is materials compatible.
Highly sensitive passive sensing devices based on surface plasmon wave interaction have been used to sense a broad variety of conditions. The condition to be sensed changes the optical properties of the sensing medium which in turn changes the resonance conditions. TM intensity also changes. The intensity change can be detected at the other end of the fiber and is a measure of the field to be sensed. Such passive sensors may be used to sense gases, chemicals, temperature, stress, strain and magnetic fields, for example. These types of sensors are discussed in "Fiber-Optic Sensor Based on Surface Plasmon Wave Resonant Coupling", Poster Paper: Meeting of OSA (Opt. Soc. of Amer.) October, 1988, Santa Clara, Calif. A prism-based surface plasmon sensor is disclosed in U.S. Pat. No. 4,844,613.
It can be seen that there are numerous advantages of surface plasmon wave modulators over MQW modulators. However, as also shown above, state of the art surface plasmon modulators are of the bulk-optic type, wherein a prism is used to effect coupling between a free-space light beam, whose source is usually a bulky and expensive laser such as HeNe, and the metal-dielectric interface which supports the surface plasmon wave. Bulk type systems suffer from stability and alignment problems. Furthermore, the speed advantage of SPW bulk-optic systems is not much greater than MQW type systems. A super high speed non-bulk SPW modulator is needed.